1. Field of the Invention
The invention relates to the field of electronic communications. More particularly, the invention relates to the field of Radio Frequency (RF) signal distribution.
2. Description of the Related Art
Signal distribution systems, such as RF signal distribution systems, typically are arranged in branched configurations. In a branched configuration, the signal typically originates at the center, or trunk, and the signal is distributed from the point of origin to the multiple end points of the branches. Each distribution branch from the trunk can be split into one or more smaller branches. Additionally, the smaller branches themselves may divide into even smaller branches one or more times prior to reaching a destination.
An RF communications signal distributed along a system is typically amplified several times along the various signal distribution paths prior to reaching a device at a destination. However, various factors operate to degrade the quality of the original signal prior to its reaching a device at a destination.
The quality of a signal is often measured as a ratio of the signal power to the noise power, referred to as signal-to-noise ratio (SNR). The signal power can be defined as the power of the portion of the signal that contains the desired information. The noise power can be defined as the combination of the random fluctuations that are uncorrelated to the information power and the undesired distortion products. The undesired distortion products can be defined to be those signal components that do not replicate the original information. Although the noise power is here used to mean the combination of the uncorrelated signals with the distortion products, other noise definitions can be used to gauge the signal quality. For example, the noise power can be represented by only the uncorrelated signals, or the noise can be represented by the uncorrelated signals and some of the distortion products.
The signal to noise ratio (SNR) typically measures the ratio of the signal to the noise and distortion experienced by the signal. Thermal noise is often a large contributor to the noise power in systems that use received radio waves as the signal source. The thermal noise level often represents the noise floor that cannot be further reduced through the use of active or passive devices. Passive losses, through passive signal division or attenuation, reduce the signal power but typically do not reduce the level of thermal noise. As such, signal attenuation can result in degradation of SNR.
Signal amplification can be used to offset some of the effects of attenuation in a signal distribution system. However, active devices, such as amplifiers, do not provide signal gain without affecting the noise and distortion that degrade the signal quality. For example, amplifiers typically introduce uncorrelated noise into the signal that further degrades the SNR. Some of the amplifier noise contribution is quantified as an amplifier noise figure, which may also be expressed as a noise factor. An amplifier's noise figure may directly contribute to signal degradation and degradation of SNR when the system operates in low power conditions close to the noise floor as is typical in a system using received RF signals.
Additionally, an amplifier may contribute distortion products that degrade the SNR. One prevalent amplifier distortion product that often contributes to in-band noise in wideband or multi-channel distribution systems is a third order intermodulation distortion product. The level of third order intermodulation distortion contributed by a particular amplifier is typically predicted based on an amplifier characteristic known as the third order intercept (IP3). The third order intercept represents a fictional operating point at which the amplifier third order distortion products would be equal in amplitude to the desired signal component. A higher amplifier IP3 is desirable. However, amplifier IP3 often correlates with amplifier power consumption. Thus, an amplifier with a high IP3 typically consumes more power than an amplifier having a lower IP3. Amplifier power consumption is of great concern because of constraints on power dissipation as well as constraints on the ability to dissipate the heat that is associated with higher power devices. Additionally, amplifiers that have low noise figures often have correspondingly low IP3.
A device, whether passive or active, can typically be designed to operate within one particular set of signal parameters at a particular location within a signal distribution system. However, signal parameters within particular location in a signal distribution system often change radically based on the number of branches before or after the device and the number of passive and active devices. placed before or after a device. Thus, a device that is optimized for one set of operating parameters may be a dominant noise source in another set of signal parameters.
What is desirable is a device and method of signal distribution that is invariant to changes in signal powers and distribution system configurations. Additionally, it is typically desirable for the device to minimize its degradation of SNR under many operating conditions. The device should also allow for the creation of numerous branches in the signal distribution path, with the inclusion or deletion of signal branches having minimal affect on other branches of the signal distribution system.